Solid-state lasers and light-emitting-diode (LED) devices can be made based on the Group III-Nitride (III-N) family of semiconductors and such devices have significantly changed fields of lighting, displays, and data storage. Efficient light extraction is important for energy efficient, high brightness light emitting diodes (LEDs). In III-N based LED devices, the typically low electrical conductivity of the p-type layer can result in much higher current injection into the active layers, and thus light generation, near the point of electrical contact.
One way of achieving high LED device efficiency is to implement a current-spreading layer that is highly transparent to light, and that low-resistance electrical contacts can be formed with the p-GaN layers. Indium tin oxide (ITO) is currently the material of choice for LED current spreading layers by various LED makers, owing to its high conductivity, and low optical absorption. There is considerable variations in the reported properties of ITO films depending on various factors such as the deposition methods, the deposition surface properties, annealing conditions, the elemental ratios of indium and tin in the films. ITO is an n-type semiconductor with a wide band-gap of 4.0 eV. However, the large amount of defects in ITO films lends to high optical absorption in the visible range, which is generally reported to be in the 650-2000 cm-1 range.
Zinc oxide (ZnO) is an optically transparent, wide band gap semiconductor. A band gap of 3.3 eV, an exciton binding energy of 60 meV, large breakdown strength, and a large saturation velocity have led to interest in ZnO as a possible candidate for use in light emitting devices and other high-power density, high-temperature semiconducting devices. In order to be used in such devices, high quality epitaxial ZnO thin films will typically be required. Many of these applications will also require the ability to produce both n-type and p-type ZnO. Unfortunately, ZnO has a strong tendency for n-type behavior and stable, reliable, and reproducible p-type ZnO has proven extremely difficult to produce. However, the tendency for high n-type conductivity combined with the high optical transparency of ZnO make it very well suited for use as a transparent conductive oxide. Like most inorganic material films used in the semiconductor industry, any ZnO films used are currently produced using vapor phase methods such as molecular beam epitaxy (MBE), pulsed laser deposition (PLD), sputtering, and metal organic chemical vapor deposition (MOCVD). However, it is also possible to produce ZnO films, including epitaxial films, using low temperature aqueous solution methods.
Because the general simplicity of the required equipment, along with the low temperatures and atmospheric pressure used, low temperature aqueous solution methods present significant cost advantages over vapor phase deposition techniques. Aqueous solution methods have been used for some time to produce ZnO powders and polycrystalline films, but more recently, it has been shown that epitaxial ZnO films can also be produced using low temperature aqueous solution methods. In general, an epitaxial film will be more transparent and have higher conductivity than a polycrystalline film of the same composition, due to the lack of grain boundaries. However, the current state of the art transparent current spreading layer technology uses a polycrystalline ITO film. Because of the dissimilar crystal structures of III-N materials and ITO, epitaxial film deposition of ITO is generally not possible. Zinc oxide, on the other hand, has the same Wurtzite crystal structure as the III-N materials used in LED devices, making epitaxial growth is possible by numerous deposition methods, including low temperature aqueous solution methods.